1. Field of the Invention
The invention relates generally to electromagnetic (EM) well logging. In particular, embodiments of the present invention relate to methods and apparatus for improving EM well logging sensitivities.
2. Background Art
During the exploration and production of oil and gas, many well logging techniques are deployed to log data of the geological formations. The data contain information that can be used to locate subsurface hydrocarbon reservoirs and to determine types and quantities of subsurface hydrocarbons. In such logging processes, a tool may be lowered into a borehole traversing a subsurface formation, either after the well has been drilled or during the drilling process. A typical logging tool includes a “sonde”, that emits, for example, acoustic or EM waves to interact with the surrounding formation. The signals produced from such interactions are then detected and measured by one or more sensors on the instrument. By processing the detected signals, a profile or log of the formation properties can be obtained.
Logging techniques known in the art include “wireline” logging, logging-while-drilling (LWD), measurement-while-drilling (MWD), and logging-while-tripping (LWT). Wireline logging involves lowering an instrument into an already-drilled borehole at the end of an electrical cable to obtain measurements as the instrument is moved along the borehole. LWD and MWD involve disposing an instrument in a drilling assembly for use while a borehole is being drilled through earth formations. LWT involves disposing sources or sensors within the drill string to obtain measurements while the string is being withdrawn from the borehole.
FIG. 1 shows a general illustration of a typical drilling rig with a drill string carrying a downhole logging tool in a borehole. The rotary drilling rig shown in FIG. 1 comprises a mast 1 rising above the ground 2 and is fitted with a lifting gear 3. The lifting gear 3 has a crown block 7 fixed to the top of the mast 1, a vertically traveling block 8 with a hook 9 attached, a cable 10 passing around blocks 7 and 8 to form on one side a dead line 10a anchored to a fixed point 11 and on the other side an active line 10b that winds round the drum of a winch 12. A drill string 4 formed of several segments of hollow drilling pipes connected end-to-end is suspended from the hook 9 by means of a swivel 13, which is linked by a hose 14 to a mud pump 15. The mud pump 15 pumps drilling mud into the well 6, via the hollow pipes of the drill string 4 and out of the bit 5 to float the rock cuttings out of the well 6. The drilling mud may be drawn from a mud pit 16, which may also be fed with surplus mud from the well 6. The drill string 4 may be elevated by turning the lifting gear 3 with the winch 12. When raising or lowering drill pipes, the drill string 4 needs to be temporarily unhooked from the lifting gear 3, during which the weight of the string 4 is supported by wedges 17. The wedges 17 are anchored in a conical recess 18 in a rotating table 19 that is mounted on a platform 20. The lower portion of the drill string 4 may include one or more instruments 30 for investigating downhole drilling conditions or for investigating the properties of the geological formations. In the case of sonic logging, the instrument 30 may include at least one transmitter and a plurality of receivers.
Variations in the height h of the traveling block 8 during the raising cycle of the drill string operations are measured by means of a sensor 23 which may be an angle-of-rotation sensor coupled to the faster pulley of the crown block 7. The weight applied to the hook 9 may also be measured by means of a strain gauge 24 inserted into the dead line 10a of the cable 10 to measure its tension. Sensors 23 and 24 are connected by lines 25 and 26 to a processing unit 27 having a clock incorporated therein. A recorder 28 is connected to the processing unit 27, which is preferably a computer. In addition, the downhole tool 30 may include a processing unit 30a. The downhole processing unit 30a and/or the surface processing unit 27, which may include a memory, may be used to perform the data analysis and determination of formation properties.
For downhole tools, EM logging tools are among the widely used. EM logging tools are implemented with antennas that are operable as transmitters and/or receivers. The antennas are typically solenoid coils. Referring to FIG. 2, a coil 211 is shown comprising of insulated conducting wires having one or more turns wound around a support 214. During operation, the coil 211 may function as a transmitter antenna when it is energized with an alternating current or an oscillating electrical signal 212. The transmitter antenna emits EM waves through the borehole mud and into the surrounding earth formation. The coil 211 may also function as a receiver antenna that collects EM signals carrying information about the interactions between the EM waves and the mud/formation.
The coil 211 carrying a varying current 212 will produce a magnetic dipole having a magnetic moment. The strength of the magnetic moment is proportional to the electric current in the wire, the number of turns of the wire, and the area encompassed by the coil. The direction and strength of the magnetic moment can be represented by a vector 213 in a direction parallel to the longitudinal axis of the coil. In conventional induction logging instruments, the transmitter and receiver antennas are mounted with their axes aligned with the longitudinal axis of the instrument. Thus, these tools are implemented with antennas having longitudinal magnetic dipoles (LMD). When an LMD antenna is placed in a borehole and energized to transmit EM energy, the induced electric currents flow around the antenna in the borehole and in the surrounding earth formations, and no net current flows up or down the borehole.
Recent EM well logging tools have tilted or transverse coils, i.e., the coil's axis is not parallel with the longitudinal axis of the support. Consequently, the antenna has a transverse or tilted magnetic dipole (TMD). The TMD configuration permits a tool to have a three-dimensional evaluation capability, such as information about resistivity anisotropy or locations and orientations of dips and faults. In addition, directional sensitivity of the data can be used for directional drilling. Logging instruments equipped with TMD-antennas have been described in U.S. Pat. Nos. 6,147,496, 4,319,191, 5,757,191, and 5,508,616. Under certain conditions, a TMD-antenna may cause a net current to flow up or down the borehole. Some TMD-antennas are configured with multiple coils. For example, a particular TMD-antenna design includes a set of three coils, and such an antenna is known as a triaxial antenna.
In wireline applications, the antennas are typically enclosed in a housing made of tough non-conductive materials such as a laminated fiberglass material. In LWD applications, the antennas are generally encased into a metallic support so that it can withstand the hostile environment and conditions encountered during drilling. Alternatively, logging instruments may be made of composite materials, thus, providing a non-conductive structure for mounting the antennas. U.S. Pat. Nos. 6,084,052, 6,300,762, 5,988,300, 5,944,124, and UK Patent GB 2337546 disclose examples of composite-material-based instruments and tubulars for oilfield applications.
Induction logging is a well-known form of EM logging. In this type of logging, induction tools are used to produce a conductivity or resistivity profile of earth formations surrounding a borehole. U.S. Pat. Nos. 3,340,464, 3,147,429, 3,179,879, 3,056,917, and 4,472,684 disclose typical well logging tools based on induction logging.
A conventional induction logging tool or “sonde” may include a transmitter antenna and a receiver antenna. Note that the designation of a transmitter and a receiver is for clarity of illustration. One skilled in the art would appreciate that a transmitter may be used as a receiver and a receiver may also be used as a transmitter depending on the application. Each antenna may include one or more coils, and may be mounted on the same support member or on different support members, i.e., the transmitter antenna and the receiver antenna may be on different tool sections. The antennas are axially spaced from each other in the longitudinal direction of the tool.
In use, the transmitter antenna is energized with an alternating current. This generates an EM field that induces eddy currents in the earth formation surrounding the borehole. The intensity of the eddy currents is proportional to the conductivity of the formation. The EM field generated by the eddy currents, in turn, induces an electromotive force in one or more receiving coils. Phase-locked detection, amplification, and digitization of this electromotive force signal determines the amplitude and the phase of the voltage on the receiver coil. By recording and processing the receiver voltages, an earth formation conductivity profile can be obtained. U.S. Pat. No. 5,157,605 discloses an induction array well logging tool used to collect the voltage data.
In principle, a conductivity profile may be obtained by simply measuring the voltages on the receiver. In practice, the receiver voltages are not only affected by “true” signals traveling through the formation, but are also affected by a direct coupling between the transmitter and the receiver. It is well known that the sensitivity of measurements obtained from induction-type loggings are adversely affected by the direct transmitter-to-receiver couplings (mutual couplings).
Mathematically, the amplitude and phase of the received signal voltage may be expressed as a complex number (i.e., a phasor voltage). Accordingly, the apparent conductivity σa (as measured by a receiver induction array) is expressed in terms of its real and imaginary parts,σa=σR+iσx.The real part σR represents the true signal from the earth formation, while the imaginary part σx includes direct coupling that may be several orders of magnitude larger than the value of σR, when the array is unbalanced. This can be seen from a well-known formula describing the conductivity measured by a two-coil (one transmitter and one receiver) array, when the transmitter is simplified as a point dipole,
                                          σ            α                    =                                                    σ                R                            +                              i                ⁢                                                                  ⁢                                  σ                  X                                                      =                                          -                                                      2                    ⁢                    i                                                        ω                    ⁢                                                                                  ⁢                    μ                                                              ⁢                                                                    (                                          1                      -                      ikL                                        )                                    ⁢                                      ⅇ                                          ik                      ⁢                                                                                          ⁢                      L                                                                                        L                  2                                                                    ,                            (        1        )            where ω is the frequency, μ is the magnetic permeability of a (homogeneous) medium, k2=iωμσ, σ is the conductivity of the medium, and L is the transmitter-receiver spacing. Defining a skin depth asδ=√{square root over (2/(ωμσ))}(so that k=(1+i)/δ)and expanding σa using the powers of L/δ, one obtains:
                                          σ            R                    +                      ⅈ            ⁢                                                  ⁢                          σ              X                                      =                  σ          -                                    2              ⁢              i                                      ω              ⁢                                                          ⁢              μ              ⁢                                                          ⁢                              L                2                                              -                                                    2                ⁢                L                ⁢                                                                  ⁢                σ                                            3                ⁢                                                                  ⁢                δ                                      ⁢                          (                              1                -                i                            )                                +                      O            ⁡                          (                                                L                  2                                ⁢                                  /                                ⁢                                  δ                  2                                            )                                                          (        2        )            The first term on the right-hand side of Eq. (2) is the formation conductivity σ of interest. The second term, −2i/(ωμL2), contributes to σx only. It does not depend on σ and corresponds to the direct mutual transmitter-receiver coupling that exists in the air. An out-of-balance induction array can have a very large value of σx, especially when L is small. Therefore, in order for an induction tool to achieve a high sensitivity, the induction array must be balanced to reduce the value of σx.
As illustrated in FIG. 3, an induction-type logging instrument 300 typically includes a “bucking” coil 311 in order to eliminate or reduce direct coupling between the transmitter 312 and the main receiver 313. The purpose of having two receiver coils, the main coil 313 and the bucking coil 311, in a balanced configuration is to cancel the transmitter-main-coil coupling using the transmitter-bucking-coil coupling. The bucking coil 311 may be placed between the transmitter 312 and the main coil 313. Practical induction arrays have always been mutually-balanced using a bucking coil. This is necessitated by the fact that the direct coupling between a transmitter and a receiver is usually several orders of magnitude stronger than the true signals, the latter being strongly attenuated when traveling through the earth formation. The instrument's 300 longitudinal axis is represented as a dashed line in FIG. 3.
The minimum configuration for a mutually-balanced array is a three-coil array as illustrated in FIG. 3, including a transmitter 312 (T), a main receiver 313 (R1) and a bucking coil 311 (R2). In order to balance the array, the locations of the main receiver and bucking coils (Zmain and Zbuck) and the numbers of turns in these coils (Nmain and Nbuck) are chosen such that the sum of their responses is close to zero in the air. That is, the voltages on the two receiver coils satisfy the relation: VR1+VR2=0, in the air. Consequently, the responses from a logging operation will be the sum of the T-R1 coupling responses and the T-R2 coupling responses.
In theory, the required positions of the receiver coils can be calculated with high precision, even when the transmitter is a finite-size solenoid. In practice, all geometrical parameters, such as the positions (zbuck and zmain) and the radii (rtrans, rbuck, and rmain) of the coils, have finite ranges of variations. The direct mutual couplings T-R1 and T-R2 are very sensitive to even minute changes in some of the geometrical parameters, and, therefore, large variations in measured signals may result from small errors or variations in, for example, the radii of the coils. Therefore, when an EM tool is manufactured, the configuration of the coils may need to be further adjusted from the calculated configuration. In a practical configuration, both positions of the bucking coil and the main receiver coil (zbuck and zmain) relative to the position of the transmitter (z=0) would need to be carefully adjusted with a high degree of precision in order to minimize direct mutual couplings. Such balancing or adjusting could be very difficult and demanding.
One prior art method of fine-tuning the antenna is to use moveable coils so that the locations of the coils (e.g., the main receiver or bucking coils) may be altered to minimize the direct coupling. For example, if the direct coupling (reflected as residual σx) for a particular array is substantial, it can be minimized (or reduced to zero) by altering the location of an antenna, such as the bucking coil zbuck. However, as a practical matter, it is preferred that the tool or antenna has no moving parts.
An alternative method for balancing the array is to add a conductive loop near one of the coils (e.g., the receiver coil) to permit fine adjustment. Another approach is to adjust with the number of turns in the bucking coil. However, this approach is often impractical because removing or adding a single turn in a coil may produce large changes in σx. This is especially true when the distance between the bucking coil and the transmitter coil is short. Therefore, there still exists a need for new approaches to balancing induction arrays.